This invention relates to voltage-controlled oscillators useful as horizontal oscillators in television applications.
In NTSC television practice, recurrent horizontal and vertical deflection of an electron beam to form a raster is synchronized with horizontal and vertical synchronizing pulses commingled in timed relation with the information to be displayed. In order to provide continuous deflection even in the absence of applied signals, the horizontal deflection is controlled by an oscillator which is synchronized with the synchronizing pulses contained in the composite video and which free-runs in the absence of synchronizing signals. Formerly, the oscillators were locked to the synchronizing signal in a direct manner, as by injection locking. In order for injection locking to be accomplished reliably, the free-running frequency of the oscillator must be near the horizontal deflection frequency. Due to the noise problems associated with direct synchronization, indirect synchronization has become universal. In indirect synchronization, a horizontal oscillator is coupled with a phase detector and a filter in a degenerative phase-lock feedback loop, in which the oscillator output is synchronized with the temporal average of the synchronizing pulses. The phase-lock loop allows a finite phase error between the oscillator signal and the average synchronizing signal, which error increases as the loop gain of the phase-lock loop decreases. In order to reduce the phase error, it is desirable to have the free-running frequency of the oscillator as near as possible to the NTSC deflection frequency of approximately 15,734 hz. It is generally desirable to avoid the need for horizontal HOLD (oscillator frequency) adjustments. To eliminate the HOLD control the automatic frequency and phase control (AFPC) of which the phase-lock loop is a part must have reliable and predictable operation under all conditions of temperature and tolerance extremes likely to be encountered. The voltage-controlled oscillator of such a loop must be stable and have a controlled range and rate of variation.
Resistance-capacitance (RC) or sawtooth oscillators are unstable with temperature and with time by comparison with inductance-capacitance (LC) types. Crystal oscillators, such as those described in U.S. Pat. No. 4,020,500 issued on Apr. 26, 1977 to L. A. Harwood and in U.S. Pat. No. 3,054,967 are stable but are difficult to pull off-frequency enough to lock to nonstandard synchronizing signals such as those from a home-type camera or a video tape recorder. Among the various LC oscillators, those using series-resonant circuits tend to have lower quality (Q) and are therefore lossier and less stable than the equivalent parallel-resonant type. A series-resonant oscillator is described in U.S. Pat. No. 4,055,817 issued Oct. 25, 1977 to Watanabe.
Among the parallel-resonant LC circuits, those requiring reactive impedance transformation to maintain high Q, such as the capacitive voltage divider shown in U.S. Pat. No. 3,553,459 issued Jan. 5, 1971 to Siedband are undesirable, because they require more than a minimum number of components.
Greatest reliability for a complex system such as an AFPC loop results when the major portion of the loop is in the form of an integrated circuit (IC). Oscillators having a parallel-resonant tank circuit coupled between the collectors of a differential transistor pair are generally unsuitable for integrated-circuit (IC) use because they require more than a minimum number of interface terminals or connection points between the IC and the external tank circuit. Among those which reduce the number of pins required by connecting one end of the tank circuit to a reference potential otherwise required for operation of the IC, the combination of the input impedance of the oscillator and of the external load may be an impedance low enough to degrade the Q of the tank circuit.
It is desirable to prevent the transistors of the oscillator from going into a nonlinear or saturated region in order to maximize stability and minimize waveform distortion. An arrangement for accomplishing this by the use of an automatic gain control circuit is shown in U.S. Pat. No. 3,649,929 issued Mar. 14, 1972 in the name of James Thompson. The AGC circuit corrects for the tolerances of, or for temperature or time-dependent changes in the values of the various components which may tend to drive the operating point of the oscillator to a state in which nonlinearity can occur. An AGC circuit, however, requires additional circuitry, including integrating capacitors. Such capacitors are not well suited to integrated circuit use. It is desirable to have a variable-frequency oscillator which operates with the transistors unsaturated and without the need for an AGC circuit. It is also desirable to have the variable-frequency oscillator circuit use a parallel-resonant tnak circuit in which the impedance presented to the tank is high so that the Q may be controlled by the external LC components and by an external resistor, and which is substantially free of circuit loading. It is also desirable to have the circuit resonate at the center frequency of the tank when the frequency control voltage is zero in order to make the VCO characteristic symmetrical and to minimize center-frequency drift.